The present invention relates to switching voltage regulators, and more particularly to a switching voltage regulator in which circuitry is provided to reduce transients generated by reverse current surges in a commutating diode.
Switching-type voltage regulators typically produce higher levels of RFI (radio frequency interference) than dissipative-type regulators. The higher noise level of the switching regulator is a general result of the abrupt transition of the switching function. Each switching cycle of the regulator may produce a large current surge that is often accompanied by a noise transient.
A switching regulator typically employs either thyristor or transistor switching means. Although thyristors are somewhat more efficient than transistors in this application, they are also noisier with regard to RFI. The reason for this is that unlike a transistor, a thyristor cannot conduct at an intermediate level. That is to say, it cannot be turned on gradually; when the thyristor is properly biased and its gate terminal is energized, the thyristor conducts fully. Each conduction cycle of the thyristor may produce current surges which affect the operation of other circuit elements and result in the generation of RFI.
In switching regulators having an inductor in series with a load, it is usual to employ a commutating (or "free wheeling") diode to shunt the input of the inductor. In this way, the diode provides a path for the inductive load current when the regulator's switching means is in the nonconducting phase of its switching cycle. It has been observed that a large RFI transient is developed in the commutating diode each time the switching means become conductive. The transient is the result of a momentary reverse current flowing in the diode.
The reverse current flow in the commutating diode is the result of a well known phenomenon commonly referred to as the "minority carrier sweep out effect." The effect is such that a momentary reverse current flows in a forward biased diode during the interval of its transition from a conducting state to a nonconducting state.
The nature of the reverse current in the diode is that of an extremely short duration, high amplitude "spike." A large radio frequency noise transient resulting from the current spike in the diode is both radiated within the local enclosure and conducted along the AC input line. The RFI conducted by the AC line frequently results in the disruption of other circuits powered by the line.
Numerous solutions have been proposed to alleviate the problem which has been described. These proposals have included the use of commercial RFI line filters and called for additional inductors to be installed at various points in the circuit of the switching regulator. In general, the prior art solutions have proven only moderately successful. In some instances, the size of the inductors required for reducing the line transients have been prohibitively large. It is apparent, therefore, that there is a need for improved circuit means for reducing transients in switching regulators.